The present invention relates to a laser plate-making apparatus for exposing a plate material made of a photoconductive sheet using a laser beam, and developing it by an electrophotographic process.
Conventional laser plate-making apparatuses of such a type are disclosed in, e.g., Japanese Patent Disclosures (Kokai) Nos. 60-40360 and 60-043668. In the laser plate-making apparatuses described in these prior art, a plurality of convey roller pairs form a convey path for a plate material, and a charger, an exposing unit, a developing unit, and a fixing unit are sequentially arranged on the convey path.
Generally, for miniaturization of the overall apparatus, i.e., for shortening the length of the convey path, the charger, the exposing unit, the developing unit, and the fixing unit are arranged to be adjacent to each other. Therefore, a length of the convey path is set to reach one and half times the length of the photoconductive sheet as a plate material. For this reason, when the plate material passes through the exposing unit, the leading end of the plate material may reach the fixing unit, or sometimes the outside of the apparatus. At least one convey roller pair is arranged between the exposing unit and the developing unit, and at least one convey roller pair is also arranged between the developing unit and the fixing unit. Consequently, the leading end of the photoconductive sheet passes between a number of convey roller pairs during exposure. While scanning a surface of the photoconductive sheet using a laser beam (in a direction orthogonal to the convey path), the exposing unit turns on/off the laser beam in accordance with image data of an original so as to form an electrostatic latent image on the sheet. Therefore, it is required that the photoconductive sheet during exposure is conveyed at a speed as constant as possible while flatness thereof is kept. Although each roller of the convey roller pairs is made of an elastic material, the leading end of the photoconductive sheet receives a slight impact when it is clamped by each convey roller pair. As a result, the photoconductive sheet is slightly vibrated, and hence flatness thereof cannot be kept. Besides, the convey speed slightly changes when the leading end of the photoconductive sheet is clamped by the convey roller pair. Consequently, when an electrostatic latent image is to be formed, a slight positional error is caused, resulting in degradation in image quality.
Furthermore, the positional error is caused by other reasons. Since the developing unit stores a developing solution, it is disposed at the lowest position in the convey path. More specifically, the convey path goes down to the developing unit, goes up therefrom, and hence the convey path is bent at the position of the developing unit. The photoconductive sheet is bent when it passes through the developing unit, and hence it is difficult for the sheet to smoothly pass through the developing unit. This unsmooth passing through the developing unit slightly vibrates the photoconductive sheet. As a result, flatness of the sheet cannot be maintained and the convey speed slightly changes.
In order to keep the photoconductive sheet flat during exposure and to quickly discharge the charges neutralized by the laser beam, a conductive support base is arranged under a part of the convey path, which is irradiated with the laser beam. A large number of fine holes are formed in the surface of the conductive support base, and a suction mechanism is coupled to these holes. During exposure, the photoconductive sheet is sucked toward the support base, thereby keeping the surface of the photoconductive sheet flat. A fan blower or an air pump may be used as the suction mechanism. In any case, a size thereof is relatively large, and it does not provide an effect to satisfy the requirement for miniaturization of the apparatus.
The photoconductive sheet is designed such that an undercoat is formed under a photosensitive layer, a base (paper) is arranged thereunder, and a back coat is formed thereunder. The photosensitive layer is charged with the positive electric charge by a charger, and the positive charge thereon is neutralized upon radiation of the laser beam by the exposing unit, thereby forming the electrostatic latent image. In this case, in order to neutralize the positive electric charge, electrons generated by the laser beam need to be allowed to move freely inside the sheet to some degree. For this purpose, the sheet must have an appropriate low electric resistance. Therefore, the sheet need have appropriate moisture.
In this case, a plurality of photoconductive sheets are stored in a paper feed magazine, and arranged to be fed one by one. A conventional magazine does not have a drying prevention function. For this reason, if the magazine is kept set in the apparatus for a long period of time, the sheet is inevitably dried due to the ambient humidity, thereby increasing the resistance due to evaporation of the moisture.
Fixation is performed by drying. Conventionally, a heater is arranged near the convey path, and air is blown by a fan to perform drying. However, the air blown from the fan is not warm air, but cool air, and hence efficiency is undesirably degraded. Furthermore, flatness of the sheet is sometimes adversely affected by the blown air, resulting in a loss of flatness.
Prior to drying, some of the developing solution is squeezed out by a squeezing roller pair from the photoconductive sheet which has passed through the developing unit. If the surface of the lower roller of the squeezing roller pair is dried, toner contained in the developing solution may adhere thereto. Therefore, a portion of the developing solution should always be sprayed on the surface of the lower roller to keep it wet. However, the sheet may be splashed with the developing solution or the developing solution may be sprayed on a dryer, depending on the force or direction of the spray of the developing solution.
Conventionally, in order to regularly measure a concentration of the developing solution, a densitometer is arranged inside the developing solution tank. However, the conventional densitometer cannot be easily cleaned if a light-emitting or light-receiving face thereof is soiled. In addition, since a tungsten bulb is used as a light-emitting portion, a considerably long preparation time is required to start measurement.